javascript tutorial - [Solved-5 Solutions] Null an object - javascript - java script - javascript array
Problem:
Why is null an object and what's the difference between null and undefined ?
Solution 1:
Is checking
if ( object == null )
Do something
click below button to copy the code. By JavaScript tutorial team
the same as
if ( !object )
Do somethingclick below button to copy the code. By JavaScript tutorial team
Solution 2:
(name is undefined)
- You: What is name? (*)
- JavaScript: name? What's a name? I don't know what you're talking about. You haven't ever mentioned any
namebefore. Are you seeing some other scripting language on the (client-)side? - name = null;
- You: What is name?
- JavaScript: I don't know.
- In short; undefined is where no notion of the thing exists; it has no type, and it's never been referenced before in that scope;
nullis where the thing is known to exist, but it's not known what the value is. - One thing to remember is that null is not, conceptually, the same as false or "" or such, even if they equate after type casting, i.e.
- name = false;
- You: What is name?
- JavaScript: Boolean false.
- name = '';
- You: What is name?
- JavaScript: Empty string
Solution 3:
alert(typeof(null)); // object
alert(typeof(undefined)); // undefined
alert(null !== undefined) //true
alert(null == undefined) //trueclick below button to copy the code. By JavaScript tutorial team
Checking
- object == null is different to check
if ( !object ). - The latter is equal to
! Boolean(object), because the unary ! operator automatically cast the right operand into a Boolean. - Since Boolean(null) equals false then
!false === true.
Solution 4:
null is not an object, it is a primitive value. For example, you cannot add properties to it. Sometimes people wrongly assume that it is an object, because typeof null returns "object".
Solution 5:
- property when it has no definition, is undefined. null is an object. Its type is object. null is a special value meaning "no value. undefined is not an object, it's type is undefined.
- You can declare a variable, set it to null, and the behavior is identical except that you'll see "null" printed out versus "undefined". You can even compare a variable that is undefined to null or vice versa, and the condition will be true: